Wash and reuse slides and cover slips (unless they're broken). We are a very small school and only use about 30 coverslips a year so washing them isn't a problem. When I worked in a larger school we just binned them.
We bin both, but we only use about box a year and have a stock pile from when we used to have multiple campuses. We have not had to buy any since I started 5 years ago and I don't think we will have to buy any in another 5 years.